Batteries are called canned energy, and it is important that they can be handled safely. In order to ensure safety even at the time of abnormalities such as overcharge and short circuit, a protection circuit such as a fuse capable of detecting abnormalities of a battery and breaking the external electrical connection of the battery is used to control the battery. Also, a mechanism for breaking an electrical connection by utilizing an abnormally increased internal pressure of a battery itself is proposed as well.
As such a secondary battery, Patent Literature 1 discloses a secondary battery in which a non-aqueous electrolyte contains a redox shuttle agent capable of causing a reversible oxidation-reduction reaction at a higher potential than a positive electrode active material and a gas generating agent capable of generating gas when a predetermined battery voltage is exceeded. Patent Literature 1 further states that a viscosity modifier capable of suppressing a decrease in the viscosity of the non-aqueous electrolyte resulting from a temperature increase is contained. Also, Patent Literature 2 discloses that a non-woven fabric of cellulose or the like having a pore size of about 0.1 μm to 10 μm is used as a separator in order to achieve stable charge-discharge characteristics that do not allow a short circuit resulting from a separator being thermally melted or shrunk. Moreover, a low-molecular gelling agent is contained in an electrolytic solution to improve the size of pores specific to a non-woven fabric and the non-uniformity of pore size distribution. Patent Literature 3 discloses an overcharge preventing mechanism by means of a redox shuttle agent added to an electrolyte solution.